1. Field of the Invention
The present invention relates to a developing device which develops an electrostatic latent image formed on an image-carrying member such as a photosensitive member or the like in an image forming apparatus such as a copier, printer and the like, and more specifically relates to a developing device of a type which transports developer via a developer-carrying member such as a developing sleeve disposed opposite an image-carrying member to develop said latent image utilizing the action of an alternating current field between a developer-carrying member and an image-carrying member.
2. Description of the Related Art
In conventional image forming apparatus such as copiers and printers, various developing devices are used to develop an electrostatic latent image formed on an image-carrying member such as a photosensitive member and the like.
One example of such a developing device is the device shown in FIG. 6. In this device, a device body 110 provided with a cylindrical developer-carrying member 111 which has an internal magnet roller 111a is disposed opposite a rotatable image-carrying member 101, such that developer 102 accommodated within said body 110 is supplied to the surface of said developer-carrying member 111 via a developer supplying member 112 such as a bucket roller or the like. Developer 102 is maintained on the surface of said developer-carrying member 111 via the magnetic force of magnet roller 111a, and in this state developer 102 is transported in the direction of the image-carrying member (i.e., in the arrow direction in the drawing) via the rotation of said developer-carrying member 111. The developer 102 maintained on the surface of developer-carrying member 111 is regulated at a uniform amount by regulating member 113, and subsequently transported to a developing region at which said developer-carrying member 111 confronts said image-carrying member 101 so as to develop an electrostatic latent image formed on the surface of said image-carrying member 101.
In the aforesaid developing device, in order to efficiently develop the electrostatic latent image formed on the surface of said image-carrying member and prevent disruption of a toner image formed on the surface of said image-carrying member 101 via the action of a magnetic brush formed by said developer 102, a direct current (DC) voltage from DC power source 114 and an alternating current (AC) voltage from AC power source 115 are overlaid and supplied to the developer-carrying member 111 so as to accomplish development via the action of the electric field generated by the overlaid AC field on the DC field in the developing region at which said developer-carrying member 111 confronts said image-carrying member 101.
When developing halftone images via the action of the electric field generated by the overlaid AC field on the DC field in the developing region at which said developer-carrying member 111 confronts said image-carrying member 101, disadvantages arise insofar as there is marked fluctuation in image density in relation to the change in surface potential in the region of surface potential of image-carrying member 101 corresponding to the halftone image, which precludes obtaining excellent halftone images and adversely affects the texture of the produced image.
Therefore, in recent years, it has been proposed to alternately repeat a first cycle (action period) to form an alternating current electric field between a developer-carrying member and an image-carrying member, and a second cycle (rest period) to halt the formation of said alternating current electric field, as disclosed in U.S. Pat. No. 4,610,531. According to the method disclosed in the aforesaid United States Patent, the final component of the alternating current electric field in the first cycle becomes an electric field component which normally pulls back the developer from the image-carrying member to the developer-carrying member.
When, however, a pair of AC electric fields are used which have inverse field directions such that the final component of an AC field normally becomes a field component which pulls back developer from an image-carrying member to a developer-carrying member as in the method disclosed in the aforesaid United States Patent, the AC field is typically at rest in a state wherein developer is pulled back from the image-carrying member to the developer-carrying member when said AC field enters the second cycle. Therefore, in the second cycle, developer is pulled back to the developer-carrying member, and as a result when the next AC field is used to accomplish development, disadvantages arise insofar as the developer is not adequately supplied from the developer-carrying member to the image-carrying member, thereby reducing the image density of the formed image. Furthermore, the texture of the formed image cannot be sufficiently improved.